The Atlantic tomcod that live in the heavily polluted Hudson River have come up with an extreme solution to deal with all the toxins surrounding them: mutate, and mutate fast. But though their super-speedy adaptation has allowed them to survive over thirty years in the dirty rivers, it may have come at a terrible price.

These tiny brown fish live just downriver of a bunch of General Electric plants, which had been steadily dumping waste in the river for decades until the practice was halted in the early 1980s. But the damage was done, and the particularly dangerous chemicals, polychlorinated biphenyls (PCBs) and dioxins, can still be found in heavy concentrations throughout the Hudson. When the dumping stopped thirty years ago, 94% of the adult tomcod had a PCB-induced tumor on their liver.

After making that initial discovery, New York University researcher Isaac Wirgin spent decades studying how the tomcod changed to meet this devastating new ecological paradigm. The results were shocking:

I started working on these fish with the hypothesis that they would be very sensitive to the toxic effects of PCBs. But the more work we did in controlled lab studies, we found that they were highly resistant to the toxic effects of PCBs and dioxin."

Yes, in the last thirty years, the tomcod have mutated, and every fish that has ever been taken out of the river in the last few years has been found to possess a gene that greatly reduces the dangers of PCBs and dioxins. It's allowed the fish to survive in waters that would be lethal to their counterparts elsewhere, but the fish have suffered in other ways. They likely grow slower than other tomcod, and they may have reduced resistance to other dangers.

But the real concern is that these fish aren't getting rid of the toxins - they're actually shunting the chemicals up the food chain:

"They serve as a prime prey for striped bass. You've got this fish that would normally be dead from PCBs or dioxin. It's alive and it's carrying around all this PCB and dioxin and it gets eaten."

The helpful mutation that has allowed the tomcod to survive involves a particular gene that, normally, would cause molecules in the fish to bind together with the toxic molecules, triggering a number of severe health problems. But the mutation discourages this action. All the fish tested possess this mutation, and 95% possess a second copy of the gene for extra protection. Only 5% of the fish in the nearby, relatively clean waters off Connecticut and Long Island possess the mutated gene.

Wirgin suspects other animals may possess the capacity for this sort of ultra-fast adaptation:

"I don't know what percent of species in a highly contaminated spot are going to be resistant. It's probably not that rare of an occurrence."